Every spring as the snow melts off the Hochschwab mountains in Austria, a lake is formed over a favourite winter hiking area. The plants and animals in the area have evolved to be able to thrive in this unusual environment.

Throughout the frozen winter months the area is almost completely dry and is used as a county park. It is a particular favourite site for hikers.

But as soon as the temperatures begin to rise in spring, the ice and snow on the mountaintops begins to melt and runs down into the basin of land below.

The park fills up with ice-cold crystal clear water, which gets its distinctive green colouring from the grass and foliage beneath.

The water levels rise from about one or two metres deep in the winter to as much as 10 metres in the late spring and early summer.

The waters are at their highest in June when it becomes a mecca for divers keen to explore the rare phenomenon, before the waters recede at the end of July.

The fauna of the “Grüner See” comprises different species of trout (Salmo), many small
animals like snails, water fleas (Daphnia pulex), small crabs and fly larvae. At least one species of mayfly exploits the environment by a combination of delayed hatching, variable growth rates, and predetermined larval development.